The long-term objective of the proposed research is to understand in detail the cellular mechanisms underlying visual transduction in vertebrate retinal photoreceptors. The specific aims are: i) to understand more about the regulation of intracellular calcium by the sodium-calcium exchanger in the photoreceptors, and to see if any difference in this regard between rods and cones may explain some of their differences in light sensitivity and response kinetics, ii) to learn more about the light-sensitive conductance in both rods and cones, and iii) to learn more about the functional constituents in the light-activated cyclic GMP reaction cascade (i.e. GTP-binding protein, the phosphodiesterase and the 48K protein), and to make a comparison between rods and cones concerning these constituents. A detailed knowledge of the visual transduction mechanism is important for understanding various disease states which affect photoreceptors, and for devising remedies for them. The methodology will involve mainly electrophysiological recordings, employing both suction-pipet and patch-clamp techniques.